246 BIOLOGICAL LECTURES. 



always displays resemblances to a number of widely separated 

 groups, not all of which resemblances can possibly be due to 

 relationship, and many of which must have been independently 

 acquired. It is easy to recognize this general principle, but it 

 is by no means easy to apply it in a given case by estimating 

 the taxonomic value of the various resemblances and differences, 

 and to distinguish the characters which are due to real affinity 

 from those which have resulted from a convergent or parallel 

 course of development. Hence it can hardly be a matter for 

 surprise that, even among competent observers, great differ- 

 ences of opinion should arise concerning the systematic posi- 

 tion of isolated groups, one writer giving special emphasis to 

 one set of characters, and another to another set. Only when 

 the ancestry of the group in question has been made out does 

 a satisfactory solution of the problem become possible. 



The indigenous selenodonts of North America first began to 

 be important in the Uinta formation, for in the strictest sense 

 of the term the Bridger has as yet yielded none, though there 

 are two or three genera which obviously represent the incipient 

 stages of the group. Their culmination, so far as numbers, 

 variety, and relative faunal importance are concerned, may be 

 regarded as falling within the White River age, though they 

 nearly held their own in the succeeding John Day. In the 

 White River we find mingled with the indigenous selenodonts 

 certain genera, like Ancodus and AntJiracotJierium, which had 

 evidently migrated from the Old World, but did not secure a 

 lasting foothold here, for no trace of them has been found in 

 the John Day. In the Loup Fork the North American type 

 of selenodonts underwent a very marked reduction, while 

 migrants from Eurasia assumed a more and more important 

 role, until, at the present, all our representatives of the group — 

 deer, antelope, sheep, bison, etc. — are descendants of Old World 

 ancestors, which, in some instances, reached North America at 

 a very recent geological date. The only survivors of the Amer- 

 ican type, the camels and llamas, are no longer found in their 

 original home, but are confined to two widely separated regions, 

 the camels to the Old World and the llamas to South America. 

 The explanation of this curious case of discontinuous distribu- 



